The present invention relates to an apparatus for detection of non-modulated signal and frequency acquisition, used in a radio communication system. That apparatus is used in an input stage of a radio receiver so that non-modulated signal at head of receive signal is detected for recognizing the presence of a receive signal and frequency acquisition for coherent detection in a receiver.
The present invention is used in a burst mode radio communication including a random access communication like ALOHA system, and/or a voice activation system.
FIG. 1 shows a signal format in those communication systems. In FIG. 1(a), the receive signal has a non-modulated signal (CW) at the head of the receive signal, and the modulated signal (MOD) follows that non-modulated signal. The modulated signal (MOD) is subject to coherent detection. The non-modulated signal (CW) is used for recognizing the presence of the receive signal, and the frequency acquisition in a receiver for coherent detection of the modulated signal (MOD). FIG. 1(b) is an example that the receive signal has two non-modulated signals (CW.sub.1, CW.sub.2) with a predetermined time interval. In case of FIG. 1(b), the detection of a receive signal is reliable since even when the first non-modulated signal (CW.sub.1) is missed because of noise, the receive signal is recognized by using the second non-modulated signal (CW.sub.2).
When a receive timing is unknown in those systems, the present invention detects a non-modulated signal at the head or the middle portion of the receive signal, and effects the frequency acquisition for coherent detection in a receiver.
Conventionally, a non-modulated signal detector, and a frequency acquisition apparatus are used separately.
FIG. 2 shows a prior non-modulated signal detector. In the figure, the numeral 9 is a bandpass filter, and 3 is a peak detector. The filter 9 is a narrow band pass filter having the center frequency equal to the frequency of the non-modulated signal which is subject to detection. When non-modulated signal appears in an input signal of the filter 9, an output level of the filter 9 is high. Then, the peak detector 3 detects the presence of a non-modulated signal by recognizing the peak output of the filter 9 on time axis, or detecting that the output level of the filter 9 exceeds a predetermined threshold level.
When a non-modulated signal to be detected has some frequency error, the filter 9 must have the pass band enough for receiving the signal with the allowable maximum frequency error. So, the larger the frequency error of a receive signal is, the larger the pass band of the filter is, and the worse the S/N of the output signal of the filter is. So, the probability of success of detecting a non-modulated signal is decreased when the frequency error is large.
FIG. 3 shows another prior art which solves the above problem, by having a plurality of bandpass filters 91 through 9n. In the figure, the numerals 91, 92, 93 .sup.. . . 9n are filters, 2 is a maximum level selector, and 3 is a peak detector. It is assumed that the filters have the different center frequency from one another so that pass band of a filter does not overlap with that of another filter. Even when the frequency error is large, one of the filters may receive the input signal. The maximum level selector 2 selects one of the filter outputs, so that the selected filter has the maximum output level. The peak detector 3 operates as is the case of FIG. 2.
An apparatus for frequency acquisition for coherent detection in a receiver must be used further in both the cases of FIG. 2 and FIG. 3, after non-modulated signal is detected, for the operation of a radio receiver. The apparatus for frequency acquisition may be a conventional phase synchronization device, or an apparatus based upon a DFT (digital Fourier Transform).
However, a prior non-modulated signal detector in FIG. 2 or FIG. 3 has the disadvantage that the probability of success of detection of non-modulated signal is worse when the frequency error is large in case of FIG. 2, and many filters must be used to complicate a hardware structure in case of FIG. 3.
Further, the delay time until the frequency acquisition is rather long, since the frequency acquisition operation starts after non-modulated signal is completely detected, in both cases of FIG. 2 and FIG. 3.